1. Field of the Invention
The present invention generally relates to a fuel cell stack. More specifically, the present invention relates to a fuel cell stack wherein a method for affording a fastening load to a fuel cell stack is improved.
2. Background of the Invention
When stacking fuel cells, a fastening load of a fuel cell stack has to be uniform over an entire range of a transverse cross section of an electrode portion of a stack in order to decrease electrode contact resistance and seal gas and water. Moreover, the fastening load must not vary substantially.
For example, even if a fuel cell surface is inclined because of a variance in a thickness of a separator, which variance may be caused by the manufacture of the separator, the fastening load of the fuel cell stack must be uniform over the entire range of the fuel cell surface. Further, a temperature of the fuel cell may repeatedly change between an environmental temperature when operation is stopped (e.g., 20° C.) and a temperature of cooling water during operation (about 80° C.), and the fastening load of the fuel cell stack may change because of the fuel cell temperature changes. Furthermore, the fastening load of the fuel stack will also change because of a creep of an electrolyte membrane and electrodes after a long period of time. However, such load changing factors must be absorbed and the fastening load of the fuel cell stack must not vary substantially.
Japanese Patent Publication HEI 8-115737 discloses a fastening structure of a fuel cell stack where, in order to uniformly fasten the fuel cell stack, a single integral elastic resilient member is disposed at a central position between a first rigid fastening plate of a first size located at a first end of the fuel cell stack and a second rigid fastening plate of a second size smaller than the first size located at the first end of the fuel cell stack, and at four corners of the second fastening plate, the second fastening plate located at the first end of the fuel cell stack and the first fastening plate located at a second, opposite end of the fuel cell stack are fastened by a bolt and nut extending in a fuel cell piling direction.
However, the above-described fastening structure of the fuel cell stack presents the following problems:
First, although the fastening structure can follow an inclination of an end surface of the pile of the fuel cells by inclining the fastening plate after deformation of the resilient member, the fastening plate cannot follow a wavy deformation of the end surface of the pile of the fuel cells because the fastening plate is a rigid plate.
Second, since the second fastening plate located at the first end of the fuel cell stack and the first fastening plate located at a second, opposite end of the fuel cell stack are fastened by a bolt extending in a fuel cell piling direction and since the bolt is not elongated, the fastening structure cannot follow a change in a length of the pile of fuel cells in the fuel cell piling direction. Therefore, the fastening structure cannot follow a thermal expansion of the pile of fuel cells nor can the fastening structure absorb a change in a load in the fuel cell piling direction resulting from a creep of the electrolyte membrane and the electrodes.